[0053] FIG. 1 shows a compartment in the bow part of a ship, into which the tank according to the invention is intended to be built. The compartment has octagonal cross sections and is defined by eight longitudinal walls 2 - 9 of the double hull of the ship, these being practically flat, adjacent via their longitudinal edges, and two transverse partitions (not depicted), respectively rear and front, which are mutually parallel and are perpendicular to the longitudinal walls. The tank compartment comprises two longitudinal walls 2 , 3 in the shape of an isosceles trapezium, forming the bottom and the roof of the tank. These two walls 2 , 3 , known as trapezoidal walls, are mutually parallel and have a common longitudinal plane of symmetry P passing through the longitudinal axis A of the compartment. These trapezoidal walls narrow from the rear towards the front of the ship. The other longitudinal walls 4 - 9 , known as lateral walls, have a rectangular shape. Each trapezoidal wall 2 , 3 defines, with an adjacent lateral wall, an oblique solid angle of intersection 10 , 11 . Two lateral walls 4 , 5 of the same width are adjacent to the bottom wall 2 , the other four lateral walls 6 - 9 , two 6 , 7 of which are adjacent to the roof wall, have identical widths, greater than the width of the previous two walls 4 , 5 . Thus, the trapezoidal wall 2 forming the bottom has a large base and a short base which are respectively longer than the long base and the short base of the trapezoidal wall 3 that forms the roof.

[0054] The two watertightness and insulating barriers at the lateral walls which, in plan view, have a rectangular shape, are produced, in the known way, by means of parallelepipedal prefabricated panels arranged longitudinally parallel to the solid angles of intersection defined by each pair of lateral walls and of running strakes as described in the aforementioned French Patent Application 2 724 623.

[0055] The production of the two watertightness and insulating barriers at the trapezoidal wall 2 forming the bottom, will now be described, those situated at the trapezoidal wall 3 forming the roof being produced in the same way.

[0056] With reference to FIG. 2 , the two secondary barriers and the primary insulating barrier are produced, on the one hand, from central panels, denoted by 12 in their entirety, aligned to form a row 14 centred on the plane of symmetry P of the wall, and, on the other hand, from two types of lateral panels, denoted by 14 and 15 in their entirety, arranged on each side of the row of central panels.

[0057] According to FIGS. 4 to 7 , each central panel 12 has practically the shape of a rectangular parallelepiped; it consists of a first plate 16 of ply surmounted by a first layer of thermal insulation 17 , itself surmounted by a rigid sheet 18 ; arranged on the sheet 18 are, on the one hand, a second layer of thermal insulation 19 covered by a second plate of ply 20 and, on the other hand, a rigid layer 21 . The subassembly 19 , 20 , 21 has, viewed in plan view, a rectangular shape, the sides of which are parallel to those of the subassembly 16 , 17 ; the two subassemblies have, viewed in plan view, the shape of two rectangles having the same centre, there being a peripheral rim 22 of constant width all around the subassembly 19 , 20 , 21 consisting of the border of the second subassembly 16 , 17 . The subassembly 19 , 20 , 21 , known as the first subassembly, constitutes a primary insulating barrier element and the subassembly 16 , 17 , known as the second subassembly, constitutes a secondary insulating barrier element. The sheet 18 which covers this first subassembly 16 , 17 constitutes a secondary watertightness barrier element.

[0058] The central panel 12 which has just been described can be prefabricated to form an assembly, the various constituent parts of which are bonded together in the arrangement indicated hereinabove; this assembly therefore forms secondary barrier elements and primary insulating barrier elements.

[0059] The rigid layer 21 consists for example of two paralellepipedal blocks 21 a , 21 b formed of bonded plates of ply, hereinafter known as ply blocks. These ply blocks are placed edge to edge in a transverse direction, leaving a relaxation gap 23 ( FIG. 5 ) between them. The second layer 19 of thermal insulation is formed of two blocks 19 a , 19 b made of a cellular plastic, such as a polyurethane foam, which is given good mechanical properties by inserting fibreglass fabrics therein. Each block of foam, covered by a second plate of ply 20 a , 20 b , is of a size practically similar to that of a ply block 21 a , 21 b . The blocks of foam are juxtaposed edge to edge, each block of foam sitting against a ply block. Relaxation gaps 24 , 25 are left respectively between the blocks of foam ( FIG. 10 ) and between a block of foam and a ply block ( FIG. 8 ). As depicted in FIG. 9 , the blocks of foam have a peripheral rim 70 . The first layer of thermal insulation 17 may consist of a cellular material identical to that of the second layer of thermal insulation. The rigid sheet 18 , which is “sandwiched” between the primary and secondary insulating barrier elements, consists of a composite material made up of three layers: the two outer layers are fibreglass fabrics and the interlayer is a metal foil.

[0060] The central panels 12 are fixed side by side to the trapezoidal wall, leaving a joining region 34 which separates the second subassemblies of two adjacent central panels. The central panels form a row 13 centred on the plane P, in which row the blocks of foam and the ply blocks alternate in the longitudinal direction, the two ply blocks 21 a , 21 b of a panel being placed downstream of the two blocks of foam 19 a , 19 b of the same panel with respect to the front transverse partition of the compartment.

[0061] Arranged on either side of this row 13 or alignment of central panels are first lateral panels 14 and second lateral panels 15 . The first lateral panels, known as standard panels, consist of prefabricated panels such as those described in the aforementioned French Patent Application. With reference to FIG. 15 , each standard panel 14 is practically in the shape of a rectangular paralellepiped; it consists of a first subassembly 26 formed of a first plate of ply surmounted by a first layer of thermal insulation, a flexible or rigid sheet and a second subassembly 27 formed of a second layer of thermal insulation covered by a second plate 28 of ply, the said second layer being arranged on the aforementioned sheet. The second subassembly 27 constitutes a primary insulating barrier element and, viewed in plan view, has a rectangular shape, the sides of which are parallel to those of the subassembly; the two subassemblies have, viewed in plan view, the shape of two rectangles having the same centre, there being a peripheral rim 29 of constant width all around the second subassembly consisting of the border of the first subassembly. The first subassembly 26 constitutes a secondary insulating barrier element and the sheet covering the first subassembly constitutes a secondary watertightness barrier element.

[0062] These standard panels 14 differ from the central panels 12 in that they have no rigid layer 21 . The primary insulating barrier elements consist solely of a second layer of insulation made of foam, covered by a plate of ply. Furthermore, the sheet between the primary and secondary insulating barrier elements may be a flexible sheet consisting of a composite material made up of three layers: the two outer layers are fibreglass fabrics and the interlayer is a thin metal foil, for example an aluminium foil approximately 0.1 mm thick. This metal foil constitutes the secondary watertightness barrier and is bonded to the layer of thermal insulation. The layers of thermal insulation of these standard panels 14 may consist of a cellular material identical to the one used to form the central panels.

[0063] These standard panels 14 are arranged in several rows and have their longitudinal axes L 1 parallel to the oblique solid angles of intersection 10 , 11 . By way of example, the oblique solid angles of intersection make an angle of the order of 15-16° with the plane P. To produce each row, the standard panels are arranged one after the other starting near the rear transverse partition and the row ends when the space remaining near the row 13 of central panels is not large enough to allow a full standard panel to be fitted. Two adjacent standard panels of one and the same row have their first subassemblies spaced apart by a joining region 35 , and two adjacent panels of two different rows have their first subassemblies edge to edge. The standard panels at the oblique solid angles of intersection will of course be mitred at an angle suited to that formed by the trapezoidal wall and the adjacent lateral wall at this oblique solid angle of intersection.

[0064] Second lateral panels known as special panels 15 are inserted between the row 13 of central panels 12 and the standard panels 14 which are at the end of each row of standard panels, so as to ensure the continuity of the two secondary barriers and of the primary insulating barrier between these. These special panels 15 are of a structure similar to that of the standard panels, comprising a first subassembly 30 and a second subassembly 31 , but having the shape of a rectangular trapezium in cross section, and are arranged side by side, leaving a joining region 36 between two adjacent second subassemblies, with their longitudinal axes L 2 perpendicular to the oblique solid angles of intersection. The two subassemblies 30 , 31 of each special panel 15 have, viewed in plan view, the shape of two rectangular trapeziums having the same centre and parallel sides, there being a peripheral rim 32 of constant width all around the second subassembly 31 , this consisting of the border of the first subassembly 30 . Each rectangular trapezium has a side which is oblique with respect to the longitudinal axis L 2 , corresponding to a face known as the oblique face 30 a , 31 a , of the first subassembly and of the second subassembly. The oblique faces 30 a of the first subassemblies of the special panels sit against the longitudinal faces of the first subassemblies 16 , 17 of the central panels 12 and their transverse faces 30 b opposite their oblique faces and perpendicular to their longitudinal axes L 2 sit against the longitudinal faces of the standard panels 14 .

[0065] These special panels 15 may be produced from prefabricated panels similar to those that make up the standard panels but cut to size at their time of installation. Furthermore, the width and length of these prefabricated panels used to produce the special panels will be tailored to suit those of the standard panels, so as to obtain a juxtaposition which is simple to install. As illustrated in FIG. 2 , the length of a standard panel is practically equal to three times the width of a special panel plus two joining regions 36 separating the special panels.

[0066] In a known way, to fix the central panels to the trapezoidal wall, there are provided, distributed uniformly along the entire perimeter of the central panels, wells 37 which are cylindrical recesses made in the peripheral rims 22 through the sheet 18 and the layer of insulation 17 down to the plate of ply 16 , as visible in FIG. 11 ; the bottom of a well therefore consists of the first rigid plate 16 of the central panel; the bottom of the well is perforated to form an orifice 38 . The trapezoidal wall carries studs which are welded at right angles to it and the free ends of which are threaded. The studs and the orifices 14 , which are large enough in diameter to allow a stud to pass, are arranged in such a way that if a central panel is offered up opposite the trapezoidal wall, the said panel can be positioned with respect to the wall in such a way that a stud faces each orifice.

[0067] It is known that the walls of a ship deviate from the theoretical surface intended for the bearing structure simply as a result of manufacturing imprecisions. In the known way, these deviations are compensated for by resting the central panels up against the bearing structure via wads of curable resin which, starting from an imperfect surface of bearing structure, make it possible to obtain cladding consisting of adjacent elements exhibiting second plates and ply panels which, in their entirety, define a surface which exhibits practically no deviation from the desired theoretical surface. When the central panels 12 are offered up in this way against the bearing structure with wads of resin interposed, the studs enter the orifices 38 and a bearing washer and a tightening nut are placed over the threaded end of the studs. The washer is pressed by the nut against the first rigid plate 16 of the panel 12 at the bottom of the well 37 . Thus, each panel 5 is fixed against the trapezoidal wall by a number of points distributed over the entire periphery of the panel, which is good from a mechanical standpoint.

[0068] The lateral panels 14 , 15 are fixed in the same way via studs present on the trapezoidal wall and wells 39 present on their peripheral rims 29 , 32 .

[0069] When such fixing has been performed in the known way, the wells in the panels 12 , 14 , 15 are plugged by inserting plugs of thermal insulation therein, these plugs lying flush with the first layer of thermal insulation of the various panels. Furthermore, a thermal insulation material forming a joint 40 and consisting, for example, of glass wool with a density of 22 kg/m ³ is fitted into the joining regions 34 which separate the first subassemblies of two adjacent central panels, and the joining regions 35 , 36 , perpendicular to the oblique solid angles of intersection, of two first subassemblies of two standard and/or special elements.

[0070] Nevertheless, while the continuity of the secondary insulating barrier has been reconstructed in this way, the same is not true of the continuity of the secondary watertightness barrier formed by the sheets covering the first subassemblies of the various panels, because this barrier has been perforated at each well 37 , 40 . To reconstruct the continuity of the secondary watertightness barrier, a band (not visible), formed of a flexible sheet, for example identical to the flexible sheet of the lateral panels, is fitted on the peripheral rims 22 , 29 , 32 that there are between two first subassemblies of two adjacent panels and the band is bonded to the peripheral rims in such a way as to close off the perforations in line with each well 37 , 39 and the joins 40 between the panels, and this may reconstitute the continuity of the secondary watertightness barrier.

[0071] As illustrated in FIG. 2 , between the subassemblies of two adjacent panels 5 there then remains a set back region situated in line with the peripheral rims 22 , 29 , 32 , this set back region having practically, by way of depth, the thickness of the primary insulating barrier. The set back regions between two central panels are filled by installing insulating tiles 41 a , 42 a , for example two of these, each consisting of a layer of thermal insulation 42 and of a rigid plate of ply 43 . Likewise, the set back regions between the lateral panels 14 , 15 and between the standard panels and the special panels are filled with tiles 44 also consisting of a layer of thermal insulation 45 and of a plate of ply 46 . The insulating tiles 41 , 44 have a dimension such that they completely fill the region situated above the peripheral rims of two adjacent panels, these insulating tiles being bonded to the aforementioned bands so that, once they have been installed, their plates 43 , 46 form, with the second rigid plates of the lateral and central panels and the top faces of the ply blocks 21 a , 21 b of the central panels, a practically continuous surface capable of supporting the primary watertightness barrier.

[0072] There then remains for the primary watertightness barrier to be fitted, which will be assembled on this practically continuous surface. To do this, there is provided, at the time of manufacture of the central panels 12 , a first longitudinal setback 47 extending on the entire length of the top faces 53 of the ply blocks 21 a , 21 b , of the rigid plates 43 , 20 covering the second insulating layer 19 of foam, and the tiles 41 a , 41 b forming the junction between two adjacent central panels 12 and over most of the width of these, forming two rims 48 which are symmetric with respect to the plane of symmetry P. A weld support 49 is fixed to the transverse upper solid angle of intersection of the ply blocks of each central panel, positioned on the side of the blocks of foam. As illustrated in FIG. 15 , the weld support 49 is formed of a section piece in the shape of a L or angle bracket, consisting of two stainless steel or Invar, preferably Invar, flanges 50 , 51 welded at right angles to each other. According to FIGS. 8 and 13 , a first flange 50 is fixed against the lateral faces 52 of the ply blocks facing the second layer 19 of insulation of the panel, and the second flange 51 is fixed against the top face 53 of the ply blocks. The top face 53 and the transverse face 52 mentioned above each have a transverse setback 54 , 55 in which one of the flanges is housed so that the first flange 50 via its top face forms a continuous surface with the bottom of the first longitudinal setback 47 . Furthermore, a chamfer 56 is formed at the said solid angle of intersection so as to leave a large enough space to accommodate the weld 57 that joins the two flanges of the angle bracket together. The angle bracket is fixed by screws 60 , screwed into the ply blocks. The flanges have a collection of holes 58 , uniformly distributed, for the passage of the screws, each hole having a conical flared portion which houses the screw head. In this embodiment, the angle bracket housed in the transverse setbacks runs transversely and beyond the first setback 47 .

[0073] According to FIG. 12 , two second longitudinal setbacks 61 are provided in the first setbacks 47 . These second setbacks are arranged symmetrically with respect to the plane of symmetry P, some distance from the rims 48 formed by the first setbacks. Housed in these second setbacks are thermal protection bands 62 intended to protect the underlying elements when the running strakes are welded, as described hereinafter.

[0074] As visible in FIG. 5 , these bands 62 do not cover the transverse setbacks 35 .

[0075] According to FIG. 4 , central strakes 63 made of rectangular Invar plate with a thickness of the order of 1.5 mm are fixed to the angle brackets 49 , by welding the transverse edges of each strake to the angle brackets 49 of two adjacent central panels. The strakes are entirely housed in the first longitudinal setback 47 which means that their upper face which faces the inside of the tank lies flush with the face of the rims 48 . The central strake known as the end strake, arranged at the front end of the row 13 , is fixed by one of its transverse edges to the angle bracket of the last central panel of the row 13 and is fixed by its other transverse edge to a rigid corner structure fixed to the bearing structure at the solid angle of intersection formed by the trapezoidal wall and the front transverse partition. The corner structure used may be of the type described in French Patents 2 709 7258 and 2 780 942. The longitudinal edges of the central strakes 63 are also screwed to the panels and the underlying blocks via screws 64 passing through uniformly spaced holes in each strake. The holes for the passage of the screws are made by punching, so as to form recesses able to accommodate the heads of the screws 64 so that these heads do not protrude. Two third longitudinal setbacks 65 , arranged symmetrically with respect to the plane P are provided in the second longitudinal setbacks 61 so as to house the material upset by the punching of the holes and corresponding to the aforementioned recesses. The screws 64 are arranged beyond the protective bands 62 with respect to the plane P, each third setback 65 running transversely between a protective band and the outer longitudinal edge of the second setback in which the said band is placed.

[0076] Running lateral strakes 66 are assembled on the standard panels 14 and special panels 15 on each side of the central strakes. In a known way, provision is made for there to be formed, at the time of manufacture of the standard panels, in their plate 28 , slots 67 a arranged parallel to the longitudinal axis L 1 of the panel and having a T-shaped cross section, the web of the T being perpendicular to the face of the plate which faces the inside of the tank, and the two flanges of the T being parallel to the said face.

[0077] The special panels comprise slots 67 b of identical shape to that of the standard panels but arranged at right angles to their longitudinal axes L 2 . These slots are spaced apart by a distance equal to the spacing between the slots 67 a of the standard panels and are arranged in the continuation of the slots 67 a of the standard panels. Likewise, the slots 67 c are provided on the plates 46 of certain tiles 44 for joining the lateral panels and the central panels and the special lateral elements. FIG. 16 depicts an example of a slot 67 c running along the tiles that form the junction between the special panels and the central panels.

[0078] Placed in these slots 67 a - c is a weld support 68 consisting, in the known way, of a section piece having a bracket-shaped cross section, one of the flanges of the bracket being welded to the turned-up edges 66 a of two adjacent running strakes, while the other flange is engaged in the part of the slot which is parallel to the mean plane of the plates 28 , 33 , 46 . In the known way, the strakes consist of Invar plates, for example 1 mm thick. The weld support can slide inside the slot which means that a sliding joint is thus produced which allows relative displacement of the running strakes with respect to the rigid plates 28 , 33 , 46 which support them.

[0079] Each plate of a standard panel 14 comprises two parallel slots 67 a spaced apart by the width of a running strake and arranged symmetrically with respect to the longitudinal axis 41 of the panel. The panels are sized in such a way that the distance between two adjacent weld flanges fitted in two adjacent panels is equal to the width of a strake; it is thus possible to position a strake in line with the central region of each plate and a strake between the two strakes, which cover the central regions of two adjacent panels.

[0080] The ends of the running strakes 66 , thus mounted to slide parallel to the oblique solid angles of intersection, are cut off to exhibit an oblique edge 69 , parallel to the plane of symmetry P. The running strakes partially cover the central strakes 63 which means that their oblique edges 69 are arranged beyond the screws 64 that fix the central strakes, and the oblique edges 69 are welded to the top face of the central strakes which faces towards the inside of the tank, along a weld line parallel to the plane P. The turned-up edges 66 a of two adjacent lateral running strakes are welded directly to one another at the end parts of the running strakes which extend beyond the joining tiles 44 and which partially cover the central strakes and all of the rim 48 . According to the embodiment of FIG. 17 , the oblique edges 69 of two adjacent running strakes 66 are cut in the region where their adjacent longitudinal turned-up edges 66 a meet the oblique edges 69 so that the weld line in this region has the shape of a saw-tooth, of which a part 69 a is practically perpendicular to the turned-up edges 66 a of the adjacent strakes.

[0081] During this welding operation, the bands 62 arranged under the central strakes afford thermal protection to the various underlying elements. The running strakes 66 covering the rims 48 and the fixing screws 64 ensure the continuity of the primary barrier. The second flange 51 of the Invar angle bracket also makes it possible to ensure the continuity of the primary watertightness barrier between the transverse edges of two adjacent central strakes.

[0082] The behaviour of the primary watertightness barrier at the trapezoidal wall upon the filling of the tank will now be described with reference to FIG. 4 . The various elements described hereinabove and that make up the wall of the tank according to the invention are mounted on the bearing structure empty, at an ambient temperature generally of between 5 and 25° C. and at atmospheric pressure. When the tank is filled with liquid methane at a temperature of about −160° C., the running strakes, although they have a very low coefficient of contraction, contract tangibly upon contact with the liquefied gas. Because the running strakes are fitted without being fixed to the surface of the lateral panels, the longitudinal thermal tensile forces F in each running strake are transmitted to the central strakes 63 to which they are welded. The central strakes, fixed to the central panels, allow part of the longitudinal component F _L of these forces to be taken up by the trapezoidal wall to which the central panels are fixed. Furthermore, given that the layers of insulation of the central panels are compressible, part of this longitudinal component is transmitted to the front partition of the compartment to which the central strake at the end of the row is fixed by a rigid corner structure. The distribution of the reaction of the longitudinal component F _L of these forces between the trapezoidal wall and the front transverse partition can be adjusted according to the flexibility of the cellular material used to make the insulating layers.

[0083] As the running strakes are arranged symmetrically with respect to the plane of symmetry P, the transverse components F _T of the said tensile forces completely or to a large extent cancel each other out.

[0084] It should be noted that each central strake is fixed to two angle brackets, each fixed to a rigid layer of a central panel, which ensures a strong securing of the central strakes and good distribution of tensile forces on the bearing structure. Furthermore, a more rigid structure is generally provided along the axis of the ship, particularly for putting the ship into dock or onto a slipway when it rests on its keel. The ships for this purpose have a reinforced central part between the double hull and the hull which cannot be “ballasted”. As the row of central panels is arranged along the axis of the ship, the aforementioned tensile forces are advantageously taken up by this reinforced central part.

[0085] Furthermore, as the running strakes of the trapezoidal wall according to the invention are arranged parallel to the oblique solid angles of intersection, it is then possible, at the said solid angles of intersection, to provide a row of corner strakes which have several undulations and which are fixed to a corner structure, such as those described in French Patent Application No. 00 10704 filed on Aug. 18, 2000 by the Applicant company.

[0086] Although the invention has been described in conjunction with one particular embodiment, it is obvious that it is not in any way restricted thereto and that it comprises all technical equivalents of the means described together with combinations thereof where these fall within the scope of the invention.